This invention relates generally to the sensing of electric charge in a relative sense and indicating quantatively a change in potential for large bodies such as the earth. U.S. Pat. No. 4,130,798 by Arthur H. MacLaren discloses a meter for detecting and indicating variations in such electric charge. That patent also points out that a continuous fair weather atmospheric electric current condition exists with a vertical potential gradient from the earth to the upper atmosphere and that various properties of this electrical condition can be measured. Changes in this electrical condition are described in that patent for predicting weather conditions.
More specifically, that patent discloses a method for sensing the charge on the earth by repeated measurements sensing quantatively changes in that charge level. To practice the method, an apparatus is disclosed consisting of a vertical quartz tube filled with mercury and connected at its lower end with a reservoir which can be varied in height so as to vary the height of the mercury column in the quartz tube. An electrode on the outside surface of the quartz column is connected to an electrometer, the low terminal of which is connected to the earth. A measurement, sensing relative charge level, is made by first short circuiting the electrometer thus stabilizing the apparatus at the potential of the earth. The level of mercury is then caused to move back and forth past the position of the electrode on the mercury column with the electrometer open-circuited during the passage of the mercury in one direction. When open-circuited, the electrometer provides a potential indication which is a direct measure of the charge induced on the electrode on the tube. A plurality of such measurements provide a quantitative indication of the change in the level of the charge of the earth. These changes in turn have been correlated to changes in weather conditions, and thus are said to be useful in predicting weather changes.
The above-referenced patent is dealing with changes in the electrostatic field above the earth's surface. It has now been recognized that in addition to this above ground field, which is mostly negative, a lower, mostly positive field extends below the earth's conductive surface. This in turn has lead to the development of a meter for measuring the changes of this lower electrostatic field and to the recognition that such changes are useful in the predicting of earthquakes and other activity below the earth's surface.